The present invention relates to devices for emitting X-rays. More specifically, the invention relates to a novel X-ray source for applications in radiology, and in particular medical radiology.
Current X-ray sources employ tubes containing a gas at a very low pressure (of the order of 10xe2x88x929 torr), in which a potential difference of about 50 to 100 kilovolts between a cathode and a target makes it possible to generate an electron flux moving at high speed in order to strike the target and emit X-rays. Their operation requires a high-tension supply, so these systems are by nature fixed, heavy and bulky. In addition, the presence of the transformers needed for the high-tension supply makes the total cost of the systems high, and imposes particularly restrictive operational and maintenance procedures.
A principle of X-ray emission by ECR (Electron Cyclotron Resonance) is known, which differs from that used by existing tube systems., and the implementation of which does not require a high-tension supply. This principle, which has been described in an article published in 1990 by Messrs. Garner et al. In [sic] Review of Scientific Instrumentation, is schematically as follows: under certain conditions, electrons emitted by a heated filament in a cavity, and exposed to microwave radiation, go into resonance so as to create a plasma in which, having reached high energy levels, the said electrons move at high speed. If a solid target is placed in the path of these high-energy electrons, the target can emit X-rays. By creating a magnetic field whose mid-plane intersects the volume of the cavity, using two magnets placed symmetrically with respect to the cavity, paths of high-energy electrons re observed which have the general shape of a ring lying substantially within the said mid-plane. The target can then be placed in this plane, in order for it to intercept high-energy electrons, so giving rise to X-ray emission. The emitted X-rays are then filtered by a thin window, made of a material such as glass, which is capable of guaranteeing that the cavity is gas-tight but which allows X-rays to pass through.
The implementation of this principle would have very considerable advantages with regard to the current tube sources, namely; a high-tension electrical supply source would no longer have to be provided, which would make the system safer, simpler to use, more economical and considerably more compact.
Attempts have been made to produce X-ray sources based on the principle of emission by ECR. These attempts, described in particular in patents U.S. Pat. Nos. 5,323,442, 5,282,899 and 5,327,475, have made it possible to produce X-rays, but in quantities and at energy levels which appear inadequate to envisage operational use, especially in medical radiology. In particular, certain applications of medical radiology require an X-ray flux to be produced with sufficient density and at an energy close to 100 keV, and the abovementioned attempts do not seem capable of producing such an X-ray flux.
A device is also known, from patent U.S. Pat. No. 5,355,399, which operates according to the same principle as the devices in the abovementioned patents and which emits X-rays, and a method associated with this device is known for the production of X-ray plates. However, in the latter device, the target, which lies in the mid-plane of the magnetic field, disturbs the generation of high-energy electrons, which results in limiting the output of X-rays emitted by the device. This has a direct effect on the operation of the device and on the quality of the images, since a low X-ray output entails long exposure times for the subject to be X-rayed, which, on the one hand is restricting in practice and, on the other, decreases the image resolution obtained because of movements by the subject during the exposure time. This major limitation prohibits the operational exploitation of the device. Thus, to date, there is no genuinely operational source of X-rays using ECR, although such a source would be transportable by virtue of its compactness, thus considerably broadening the field of application of X-ray imaging machines (use in medical imaging in environments which are difficult to access, for example at the site of an accident, or in industrial imaging, for example in submarines, aircraft or factories).
One object of the present invention is to enable a radiology device according to the principle of X-ray emission by ECR to be produced in such a way as to provide an X-ray output which is sufficient, in particular, to produce images of a quality at least equal to that of the current plates produced by tube machines, while using a compact and transportable X-ray source and without employing a high-tension supply.
A second object of the invention is to enable an X-ray source to be produced, whose response time to a control signal is short enough to produce fixed or moving X-ray images,
A third object of the invention is to enable stereo X-ray images to be produced and to provide X-ray images in relief.
To achieve these objectives, the present invention proposes, according to a first aspect, an X-ray emission device comprising a microwave source, a resonance chamber containing a hermetically sealed volume of gas, a magnetic structure defining a geometrical electron-confinement zone in which electrons move at high speed and at least one target placed in an electron path in order to emit X-rays, characterized in that the or each target is offset with respect to a mid-region of the geometrical confinement zone.
Preferred, but non-limiting, aspects of the device according to the invention are as follows:
the magnetic field is symmetric and the mid-region of the geometrical confinement zone is a plane;
the magnetic structure comprises at least one pair of permanent magnets placed on either side of the resonance chamber;
the magnetic structure comprises coils placed on either side of the resonance chamber;
means to vary the intensity of the current in the coils are provided;
the means to vary the intensity of the current in the coils are capable of varying the intensity slowly enough to allow the electrons to keep the ratio B/xcex3 almost constant, where B is the value of the magnetic field and xcex32=1+v2/c2, v being the electron speed, c the speed of light and xcex3 the electron mass;
the device comprises means to alter the configuration of the magnetic structure so as to vary the energy of the X-rays emitted;
the device comprises means to alter the position of the target or targets so as to vary the energy of the X-rays emitted;
the microwave source comprises means to modulate the microwave emission;
the device comprises two targets;
the positions of the two targets are symmetric with respect to the central region of the geometrical confinement zone.
According to a second aspect, the invention also provides a radiography system comprising a device for emitting X-rays according to one of the aspects described hereinabove and comprising two targets; means to form two X-ray images of the same object from two different angles and means to reconstruct a stereo X-ray image of the said object.